This application claims the benefit of EP Application No. 1309335.9, filed Nov. 2, 2001, in the Europe Intellectual Property Office, the disclosure of which is incorporated herein by reference.
The invention relates to a dual-parameter optical waveguide grating sensing device and to a dual-parameter optical waveguide grating sensor including such a device.
Fibre Bragg gratings are fabricated by exposing the core of a photosensitive optical fibre to a UV laser interference pattern, thereby inducing a permanent, periodically varying, change in its refractive index. The photosensitivity of an optical fibre may be increased by hydrogenating the fibre. In this specification hydrogenation of an optical fibre is understood to mean loading an optical fibre with hydrogen or an isotope of hydrogen, such as deuterium. A number of different FBG types have been distinguished, characterised by markedly different spectral and thermal behaviours, originating from the UV excitation of several distinct physical mechanisms.
During the typical fabrication process, the resonant wavelength of the grating increases with UV exposure duration, indicative of the induction of a positive change in refractive index. The resulting FBG has come to be referred to as a Type I grating, and is the type most commonly fabricated using either free-space holographic or phase mask exposure techniques. Type I gratings may be fabricated in hydrogenated or non-hydrogenated optical fibre.
For a Type I grating fabricated in non-hydrogenated fibre, further UV exposure causes the Type I grating growth to become saturated. As the UV exposure continues, the observed reflectivity of the grating decreases almost to zero before increasing again as formation of the so-called Type IIA grating commences. The resonant wavelength of the grating decreases during this stage of the exposure, indicating a negative induced refractive index change. When a Type I grating is fabricated in hydrogenated optical fibre, saturated growth and erasure of the Type I grating is followed by formation of a regenerated optical fibre grating, named a Type IA grating, as disclosed in our co-pending European application No. 01308804.2 field on 16 Oct. 2001.
Due to their wavelength division multiplexing capabilities, fibre Bragg gratings (FBGs) have been the subject of numerous investigations for optical sensing. However, the simultaneous sensitivity of the resonant wavelength of the FBG to temperature and strain has complicated independent measurement procedures for these two parameters. To overcome the problem of cross-sensitivity, several methods have been proposed and investigated including dual-wavelength superimposed gratings, utilisation of the first- and second-order diffraction from a single FBG, a hybrid FBG/long period grating (LPG), an FBG superimposed on a polarisation-rocking filter, and spliced FBGs inscribed in different fibres. All of these methods are, in effect, dual-grating methods and although they have achieved parameter discrimination, most of them are far from suitable for practical use due to either insufficiently large differentiation of the thermal and strain coefficients between the gratings, or over-complex signal interrogation and processing requirements. Additionally, in the case of the otherwise attractive FBG/LPG structures, the broad spectral response of the LPG imposes severe limitations on multiplexing.
According to a first aspect of the present invention, there is provided an optical waveguide grating sensing device for a dual-parameter optical waveguide grating sensor, the sensing device comprising:
a first optical waveguide grating of a first resonant wavelength provided in a first section of optical waveguide; and
a second optical waveguide grating of a second resonant wavelength provided in a second section of optical waveguide,
the first and second gratings having different coefficients of rate of change of wavelength as a function of temperature and having substantially the same coefficient of rate of change of wavelength as a function of strain.
The first and second sections of optical waveguide are preferably first and second sections of optical fibre. The first and second sections may be sections of a single optical fibre, and may be provided substantially adjacent one another. The optical fibre is preferably Boron-Germania co-doped optical fibre.
The first and second optical waveguide gratings are desirably fibre gratings, and may be fibre Bragg gratings. Preferably, the first fibre grating is a type I fibre grating or a type IIA fibre grating. The type I fibre grating may be a type I fibre grating fabricated in hydrogenated optical fibre or a type I fibre grating fabricated in non-hydrogenated optical fibre. The second fibre grating is preferably the other of a type I fibre grating fabricated in hydrogenated optical fibre or a type I fibre grating fabricated in non-hydrogenated optical fibre, or a type IA fibre grating.
According to a second aspect of the present invention, there is provided a dual-parameter optical waveguide grating sensor comprising:
an optical waveguide grating sensing device according to the first aspect of the invention; and
interrogation means operable to determine the resonant wavelengths of the gratings and to measure any changes therein,
the first and second gratings being arranged such that, in operation, both gratings are exposed to the same temperature and strain, whereby a change in the temperature and strain experienced by the gratings causes a change in the resonant wavelengths of the gratings, and measurement of the induced wavelength changes, together with the temperature and strain coefficients of the gratings, enabling the magnitude of the changes in temperature and strain to be determined simultaneously.
The dual-parameter optical waveguide grating sensor may comprise a plurality of optical waveguide grating sensing devices according to the first aspect of the invention, each device being located at a different sensing location such that each device may be used to simultaneously determine the temperature and strain at the respective location, thereby forming a quasi-distributed sensor.
Preferably, each grating within the sensor has a different resonant wavelength. The resonant wavelengths of the gratings within spectrally adjacent sensing devices are preferably selected such that, in operation, the spectral profiles of the gratings within a first sensing device do not substantially overlap with the spectral profiles of the gratings within an adjacent sensing device.